Hearing aid circuit



April 1944- s. F. LYBARGER 2,345,761

HEARING AID CIRCUIT Filed March 15, 1943 WI f/YEJJE a.

Patented Apr. 4, 1944 HEARING AID omourr Samuel F. Lybarger, Pittsburgh, Pa., assignor to E. A. Myers 8; Sons, Mount Lebanon, Pa., a partnership consisting of Edward A. Myers, Edwin J. Myers, and Samuel F. Lybarger Application March 15, 1943, Serial No. 479,237

3 Claims.

This invention relates to wearable hearing aids, and more particularly to an amplifier circuit using three or more vacuum tubes.

The use of a self-biasing resistor to provide the C bias in an amplifier tube is well known and its advantages are clearly recognized. In large amplifiers in which indirectly heated cathodes are used, it is a simple matter to provide such a resistor for any tube without introducing common resistance into the B supply circuit. However, with battery-operated units that require directly heated filaments, unless a separate A battery is employed for the tube to be biased, the biasing resistance must be placed in the B supply circuit which is common to all the rest of the tubes in the amplifier. Because of this, regenerative or degenerative efi'ects occur which may be undesirable, particularly if the efiect is regenerative and oscillation of the amplifier takes place. The usual way to avoid such oscillation is to place a large by-pass condenser across the biasing resistor so that audio-frequencies which might be produced across the resistor by the alternating output current of one tube and carried back to an earlier tube in the circuit in such a phase relation that oscillation might occur, are efiectively short-circuited by the action of the condenser.

In a hearing aid, however, the use of such a condenser means that either the hearing aid must be quite large to accommodate it, or the condenser must be a. very small, low voltage electrolytic type which must have a capacity of severel microfarads. Large condensers are impractical for modern wearable hearing aids which are made as small as possible, and small electrolytic condensers thus far have proved to be very unsatisfactory from a life standpoint.

It is among the objects of this invention to provide a hearing aid vacuum tube amplifier circuit which permits the use of a self-biasing resistor in the common battery supply circuit of all tubes without producing oscillation, which does not require a large by-pass condenser across the bias resistor, which is extremely stable even to further increases in the B supply circuit resistance as might be caused by increased internal B battery resistance, and which has stable operation even at extremely low A battery voltages.

In accordance with this invention a hearing aid amplifier circuit, including at least three vacuum tubes with a plate coupling resistance for the first tube, has a. common B battery supply circuit for all tubes and a bias resistor in that circuit for the output tube. A decoupling resistor is connected in series with the plate coupling resistance, and a small by-pass condenser is connected between their junction and the cathode of the first tube. A screen dropping resistance is connected directly from positive of the B battery supply to the screen grid of the first tube, and a by-pass condenser connects this screen grid to the cathode of that tube. The capacity of the condenser is such that the portion of the voltage in the plate circuit of the first tube resulting from feedback voltage that is produced as a result of the use of a bias resistor and that reaches the screen grid, is greater than the feedback voltage reaching the plate of the first tube through the plate coupling resistance. The result is that the last-mentioned feedback voltage is suppressed and does not produce oscillation in the amplifying circuit. The tubes may be selected to provide a second tube that will cease amplifying before the first tube as their filament voltage decreases. This prevents oscillation at low filament voltages. Or, means may be provided for maintaining the filament voltage of the second tube sufficiently lower than that of the first tube to insure the second tube ceasing amplifying first.

The invention is illustrated in the accompanying drawing in which a circuit diagram is shown.

Referring to the drawing, a crystal microphone I, such as is generally used for hearing aid use, has one of its terminals connected to the control grid 2 of a pentode voltage amplifier tube 3. The screen grid 4 in the tube is connected by a screen dropping resistor 5 directly to the positive side of the B battery 6. This resistor is of a size to provide the correct D. C. voltage on the screen. The screen and resistor are connected to the tube filament 1 by a screen by-pass condenser 8. The plate 9 of the tube is connected by a plate coupling resistor l0 and other means, to be described later, to the positive side of the B battery, and the plate is also connected by a coupling condenser II to a volume control i2 located in the grid circuit of the second vacuum tube I3.

The screen grid M of the second tube is connected by a screen dropping resistor l5 to the positive side of the B battery. This screen grid and resistor are connected by a by-pass condenser It to the negative side of the A battery II. The size of this condenser is relatively important in determining the response of the second amplifier stage. Resistor I8 is the plate coupling resistor for the second stage, and condenser I9 is the coupling condenser feeding into a grid resistor 20 and to the grid of the output tube 2|.

The negative terminals of the A and B batteries are connected together through a biasing resistor 25. If desired, additional bias may be provided by employing one or more bias cells 26, preferably of the fixed type, between the grid resistor and the negative side of the B battery. The plate of output tube 2| has a plate choke 21 which is coupled to a crystal receiver 28 through a condenser 29 and a shunting resistor 30.

Shown near the microphone are a resistor 35, a condenser 36, and a switch 31 which provide a means for controlling the frequency response of the amplifier and thereby, in effect, act as a tone control for the unit. With the switch in such position that the resistor is shunted across the microphone, a reduction in low frequency response relative to high frequency is provided, giving a rising amplification characteristic with frequency. When the switch is on the center position, the normal response of the amplifier is obtained. ,When condenser 36 is shunted to ground, the high frequencies are reduced relative to the low frequencies, resulting in a falling amplification characteristic with frequency. Such a control has been found to provide the necessary changes to take care of different types of hearing impairments.

In accordance with this invention, there is connected in series with the plate coupling resistor l0, between it and the positive of the B battery, a decoupling resistor 40. Also, a relatively small by-pass condenser II is connected between the cathode 1 of the first tube and the junction of the decoupling and plate coupling resistors. These form a decoupling circuit between the B battery supply and the plate of the first tube and, consequently, also between the B battery supply and the grid of the second tube.

It sometimes is desirable to use a transformer in place of the choke and condenser arrangement shown in the drawing. In such a case it is often desirable to limit the amount of bias voltage produced by the bias resistor because. if this resistor is too large, more negative feedback is produced in the last stage than is desired. However, with certain types of tubes and with the amount of negative feedback limited to the desired amount in the last stage, 13113 total bias voltage may not be suflicient to give thegproper operating conditions. Additional bias maybe...

provided in the form of the small permanent type bias cell 26 mentioned above. With this arrangement, many of the advantages of using the bias resistor 25 are retained, but the gain of the last tube is not reduced more than desired.

The operation of this circuit briefly is that the D. C. plate current flowing through bias resistor 25 produces a negative voltage between the filament of output tube 21 and the end of the res stor connected to the minus side of B battery 6. This negative voltage is carried back through resistor 20 to the grid of the output tube and provides the necessary biasing on the tube to give most satisfactory operation. Resistor 25 also has the effect of stabilizing the plate current for different tubes, which, without this self-biasing,

would show large variations in plate current. It

also provides negative feedback in the last stage which reduces the distortion. These features are well known.

Because of the fact that resistor 25 is not bypassed, all of the alternating current which flows in the plate circuit of the last tube will flow through the bias resistor, resulting in an alternating voltage across it. Some of this alternating voltage will be fed back to the grid of the second tube I3 by means of the path through the B battery, the decoupling resistor 40, the plate coupling resistor in, and the volume control l2. Feedback voltage so transmitted will be of such phase that oscillation will tend to occur as a result of the bias resistor not being by-passed.

However, the amount of voltage fed back through this path will be appreciably reduced by the inclusion of decoupling resistor 40 and by-pass condenser 4| which act as a decoupling circuit. The

important thing is that with the other features that will be described later, the decoupling circuit need not be made with high values of capacity such as could be employed in a larger size amplifier where space is not so limited. For instance, a capacity of .05 microfarad at condenser AI is very adequate to provide a large reduction in this feedback voltage. The decouling resistor 40 may be on the order of 0.1 megohm.

However, at the same time that voltage tending to produce oscillation is' feeding back to the grid of the second tube, alternating voltage which was produced across the bias resistor is also going back to the screen grid 4 of the first tube 3 through the screen dropping resistance 5. This voltage is diminished somewhat by condenser 8 which is necessary to provide proper operation of the first amplifier stage but which must be made small enough so that a suitable amount of alternating feedback voltage reaches the screen. Under usual conditions it is not necessary for a hearing aid amplifier to have nearly as good response at the low frequencies as is found in larger amplifiers, because the low frequencies do not contribute appreciably to intelligibility and they do tend to overload the last amplifier stage because of their relative strength in the voice spectrum. Thus, condenser 8 can be made fairly-small so that it does not by-pass too much of the feedback volta e and yet the performance of the first stage will not be seriously impaired. It should also be mentioned that in practice most oscillation is found to occur at very low frequencies with this circuit, and since the amount of by-pass produced by condenser 8 will decrease as the frequency decreases because the impedance of the condenser rises as the frequency goes'down, then the feedback voltage becomes considerably 1arger for those frequencies at which there is a greater tendency for oscillation, which is a distinct advantage.

Alternating voltage presented at the screen grid 4 of the first tube will be amplified in that tube and will pass on to the grid of the second amplifying tube in such a phase as to counteract the undesirable feed-back voltage that tends to produce oscillation in the circuit. By choosing suitable values of dropping resistors and condensers it is possible to completely counterbalance the effect of the other voltage tending to produce oscillation without using large size bypass condensers at any point. This is extremely important because room is not available to provide the large by-pass condensers which are generally employed in larger amplifiers. Wearable hearing aids are too small for such condensers. and small electrolytic condensers give extremely poor performance and may cause a constant small drain on the B battery.

megohm; and bias resistor 25 from 1000 to 5000 ohms depending on the type of output tube and output load and on whether additional bias is being supplied by a bias cell 26. Typicalcapacities for condensers are: screen by-passcondenser 8, .001 to .01 microfarad, and decoupling condenser ll, .05 microfarad.

While the conditions as described above are true over most of the frequency band there is a possibility that the phase relations will not be correct for the extreme end of the frequency band where phase shifts produced by coupling condensers and screen by-pass condensers become large. As a result, it is found desirable to keep the amplification of the second stage somewhat lower than normal at extremely low frequencies.

This may be accomplished by reducing the size of condenser l6 or condenser I9. By doing this the amplification at low frequencies is so greatly re-- duced that even though the phase of the voltage reaching the grid of the second tube from the bias resistor would tend to produce oscillation, the amount of amplification through the second tube and the output tube back to the bias resistor is not sufficient to cause oscillation.

In the practical operation of this circuit, it is found that there is an appreciable variation in the low voltage cutoff point of the voltage amplifier tubes such as are used in the first and second stages. Thus, it can be seen that if the first tube ceases to amplify while the second tube isstill operating satisfactorily, the degenerative feedback voltage which must be produced by the amplifying action of the first tube on the voltage carried back to its screen grid will not counteract the regenerative feedback voltage applied to the grid of the second tube. Therefore, under practical conditions it is found that while very high values of the bias resistor may be used when the filament voltage is normal, oscillation is often produced, when the filament voltage is reduced appreciably below the normal value, in the event that the first tube has a higher cutoff voltage than the second tube. In order to avoid this difficulty, tubes may be selected to provide a second tube that has a higher cutoff voltage than the first one. This method has proven very satisfactory.

Instead of selecting tubes to provide a second tube having a higher value of filament cutoff voltage than the first tube, means may be put into the circuit to reduce the filament voltage of the second tube below that of the first; The amount of reduction needed will be small, .probably on the order of 0.1 to 0.2 volt, and will depend on the range of filament voltage cutoff found in the type of tubes being used. By filament voltage cutoff is meant the filament voltage at which the amplification of the tube in circuits of the type used starts to drop off to a low value or to disappear. If all tubes of a given type cut off between, for example, 0.6 volt and 0.8 volt, then if sufiicient voltage drop were provided in the second stage so that the filament voltage in this stage were 0.55 volt when that in the first stage was 0.8 volt, any combination of tubes employed would give satisfactory performance.

To accomplish this reduction in second stage filament voltage when the first two tube filaments are in series, a resistance 45 may be placed across the filament 46 of the second tube in order to insure that that tube will cease amplifying somewhat sooner than the first tube at very low filament voltages. An additional series resistor 41 may be added to the filament circuit to prevent current through the first tube from becoming excessive as aresult of the shunting resistor across the second tube. If the filaments are in parallel instead oflin series, then a single 5 resistor may be placed in series with the filament of the second tube to cut down the volt age across the terminals of this filament.

According to the provisions of the patent statutes, I have explained the principle and construction of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced 5 otherwise than as specifically illustrated and described.

I claim: 1. An amplifier circuit for a wearable hearing aid, comprising at least three vacuum tubes, the first tube having a plate and a screen grid, a plate coupling resistance for said first tube, a common B battery supply circuit for all tubes, a bias resistor in said common circuit for the output tube, a decoupling resistance in series with said plate coupling resistance, a by-pass condenser between the cathode of the first tube and the junction of said decoupling and plate coupling resistances, a screen dropping resistance connected directly from positive of said B battery supply to said screen grid, and a lay-pass condenser connecting said screen grid to said cathode and having such capacity that the portion of the voltage in the plate circuit of the first tube resulting from feedback voltage that is produced as a result of the use of said bias resistor and that reaches said screen grid is greater than the feedback voltage reaching said plate through said plate coupling resistance,

whereby said last-mentioned feedback voltage 40 does not produce oscillation in the amplifier circult.

2. An amplifier circuit for a wearable hearing aid, comprising at least three vacuum tubes, the first tube having a plate and a screen grid, a plate coupling resistance for said first tube, a common B battery supply circuit for all tubes, a bias resistor in said common circuit for the output tube, a decoupling resistance in series with said plate coupling resistance, a by-pass condenser between the cathode of the first tube and the junction of said decoupling and plate coupling resistances, a screen dropping resistance connected directly from positive of said B battery supply to said screen grid, and a by-pass condenser connecting said screen grid to said cathode and having such capacity that the portion of the voltage in the plate circuit of the first tube resulting from feedback voltage that is produced as a result of the use of said bias resistor and that reaches said screen grid is greater than the feedback voltage reaching said plate through said plate coupling resistance, whereby said last-mentioned feedback voltage does not produce oscillation in the amplifier circuit, and the second tube being adapted to cease amplifying before the first tube as the filament voltage for the tubes decreases.

3. An amplifier circuit for a wearable hearing aid, comprising at least three vacuum tubes, the first tube having a plate and a screen grid, a plate coupling resistance for said first tube, a common B battery supply circuit for all tubes, means for supplying filament voltage to the tubes, means for maintaining the filament voltage of the second tube sufficiently. lower than that of the first tube so as to insure the second tube ceasing amplifying before the first tube as the filament voltage for all of the tubes decreases, a bias resistor in said common circuit for the output tube, a decoupling resistance in series with said plate coupling resistance, a bypass condenser between the cathode of the first tube and the junction of said decoupling and plate coupling resistances, a screen dropping resistance connected directly from positive of said 10 cuit.

B battery supply to said screen grid, and a bypass condenser connecting said screen grid to said cathode and having such capacity that the portion of the voltage in the plate circuit of the first tube resulting from feedback voltage that is produced as a result of the use of said bias resistor and that reaches said screen grid is greater than the feedback voltag reaching said plate through said plate coupling resistance, whereby said last-mentioned feedback voltage does not produce oscillation in the amplifier cir- SAMUEL F. LYBARGER; 

